The basic reactions in soapmaking are quite simple. They either consist of reacting fat with an alkali to produce soap and glycerine, or to neutralize fatty acids with an alkali. On the other hand, the technology of soapmaking is quite involved, and practical soapmaking borders at times on an art because of the complex physical nature of soap and its aqueous systems. Saponification of fats is in itself an exacting operation and is illustrated by Equation 1, below: ##STR1## wherein R.sub.1 represents saturated, unsaturated, polyunsaturated, or branched aliphatic chains having C=7-19;
R.sub.2 represents saturated, unsaturated, polyunsaturated, or branched aliphatic chains having C=7-19; PA1 R.sub.3 represents saturated, unsaturated, polyunsaturated, or branched aliphatic chains having C=7-19; and PA1 R represents a mixture of R.sub.1, R.sub.2 and R.sub.3.
In this process, the soap, after saponification, is usually carried through a series of phase changes for the removal of impurities, the recovery of glycerine, and reduction of the moisture content to a relatively low level. The complex series of operations in the production of an ordinary full-boiled or settled soap is as follows: (a) reaction of the fat with alkali until it is largely saponified, (b) graining out of the soap from solution with salt in two or more stages for recovery of the glycerol produced by the reaction; (c) boiling of the material with an excess of alkali to complete saponification, followed by graining out with alkali; and (d) separation of the batch into immiscible phases of neat soap and niger, the so-called "fitting" operation. The final result is "neat" soap with a composition ranging from 60-65% soap and about 35-40% water, plus small amounts of salt and glycerine.
When fatty acids are used as the starting material, reaction with alkali is a conventional neutralization as shown in equation 2. EQU R--COOH+NaOH.fwdarw.R--COONa+H.sub.2 O EQUATION 2.
The fatty acids are usually obtained by splitting fats into fatty acids and glycerol using high pressure steam with and without the use of a catalyst. (Bailey's Industrial Oil and Fat Products, 4th Edition, Volume 1, Chapter 8, pp 99-103, John Wiley and Sons Inc., 1979.) This is followed by distillation of the crude fatty acids and neutralization of the distilled fatty acids. Selection of the proper concentration of alkali will result in the production of neat soap described above. For the production of non transparent and certain translucent soaps, the neat soap is then dried to a moisture content of 12-15%.
A breakthrough from the traditional soap-boiling processes was the advent of various continuous saponification processes which emerged after World War II. These processes fell into two main categories: those based on the continuous saponification of fats, i.e., the DeLaval, the Sharples, Mechaniche Moderne, and the Mazzoni SCN-LR processes; and those based on the continuous splitting of fats into fatty acids followed by distillation and neutralization. Typical examples are the Mazzoni SC and the Armour-Dial processes. A more complete description of these processes appears in Bailey's (Ibid, pp. 535-549), and will not be repeated here.
In spite of the development of continuous soapmaking processes, industry has heretofore been unable to adapt any of these processes to the efficient and economical production of high quality transparent soaps. Transparent soaps are traditionally prepared by the semi-boiled or by the "cold process", utilizing special fat blends. (Bailey's, Ibid, pg. 534.) They often contain additives such as sugar, glycerol, alcohol, triethanolamine and rosins. They are poured into frames, held at room temperature for periods of time, and thereafter cut into bars.
Processes for the manufacture of transparent soaps have been known for a long time, the oldest recorded product being "Pears Transparent Soap" which was first offered for sale in England in 1789.
As a point of reference, "transparent soap", as that term is used herein encompasses soaps having a wide degree of color and gloss but which are sufficiently transparent so that one with normal vision can effectively see through a toilet sized bar. Specifically, if 14 point type can be seen through a 1/4 inch thick bar of soap, that bar of soap is defined as "transparent". (Wells, F. M., Soap and Cosmetic Specialties, 31 (6-7) June-July, 1955.)
Because regular and transparent soaps traditionally have a pH of 10 or higher, and many transparent soaps often contained alcohol, they acquired a reputation of causing skin dryness. Fromont (U.S. Pat. No. 2,820,768) addressed this issue with a less alkaline transparent soap free of alcohol and based on a blend of sodium and triethanolamine soaps from tallow, coconut oil and castor oil and "superfatted" with fatty acids such as stearic acid and oleic acid. Soap manufactured under this patent was marketed under the trade name Neutrogena.RTM. and found to be exceptionally mild. The mildness of this formula has been demonstrated using the Soap Chamber Test. (Frosch, P. J. and Kligman, A. M.: The Soap Chamber Test. J. American Academy Dermatology, 1:35, 1979 and Dyer, D. and Hassapis, T. Comparison of Detergent Based Versus Soap Based Liquid Soap. Soap Cosmetic and Chemical Specialties. July, 1983). In this test, an 8% soap solution is applied to the arms of volunteers using an occlusive patch/chamber. The soaps are applied for 8 hours per day for 5 days, and the resultant damage to the skin is rated. In this testing the Neutrogena.RTM. transparent bar formula has been shown to be milder than the other bar soaps tested. In addition, this mildness has also been demonstrated in exaggerated use tests and antecubical wash test. (Principle of Cosmetics for the Dermatologist. Frost, P. and Horwitz, S., Chapter 1, pp 5-12, C. V. Mosby Company, 1982.)
Pape (U.S. Pat. No. 2,005,160) described a method for making milled transparent soap from a blend containing rosin but no alcohol or sugar. The process included "shock cooling", that is, reducing the temperature of the soap mass from 100.degree. C. to 20.degree. C. in 2 seconds.
Later, Kelly (U.S. Pat. No. 2,970,116; French Pat. No. 1,291,638; and U.K. Pat. No. 1,033,422) developed a process for making milled translucent soaps by mechanical working and milling at controlled temperatures and vacuum plodding. Though having obvious advantages over the older processes, Kelly's processes never achieved any wide scale use or success. The bars were translucent and did not achieve the transparency defined previously.
Kamer et al (U.S. Pat. No. 3,562,167) taught a batch process for making a transparent soap formulation containing specified nonionic surfactants. In addition, Lager was granted U.S. Pat. No. 3,969,259 for incorporating germicides such as 2,4,4'-trichloro-2'-hydroxydiphenyl ether (Irgasan DP 300) into transparent soap bars.
At this point in time, the production of transparent soaps worldwide remains a batch process; continuous production without serious aesthetic defects (i.e. loss of transparency) has not been obtained.
The economic desideratum still eludes the industry for, except as indicated, the production of transparent soap remains a batch by batch process and continual production without serious aesthetic defects has not been obtained.
The present invention is directed to a process for the continuous production of transparent soap while improving the economy of production, enhancing the volume and rate of production without sacrificing any of the clarity associated with batch produced bars. In addition, quality improvements, such as lighter color and greater perfume stability is obtained by this continuous process.